Question 1

Which of the following best describes the committed and rate-limiting step in heme biosynthesis in the liver?

Accepted Answer

The condensation of glycine and succinyl CoA to form $\delta$-aminolevulinic acid (ALA), catalyzed by ALA synthase (ALAS1)

Question 2

A patient presents with photosensitivity and red-brown urine that fluoresces under UV light. Laboratory tests reveal elevated uroporphyrinogen in the urine. Which enzyme is most likely deficient?

Accepted Answer

Uroporphyrinogen decarboxylase

Question 3

Which of the following statements about the regulation of hepatic ALA synthase (ALAS1) is correct?

Accepted Answer

Hemin represses ALAS1 gene expression, increases degradation of its mRNA, and decreases import of the enzyme into mitochondria

Question 4

Which of the following is a direct precursor for the synthesis of catecholamines such as dopamine, norepinephrine, and epinephrine?

Accepted Answer

Tyrosine

Question 5

A patient with hemolytic (prehepatic) jaundice is expected to have which of the following laboratory findings?

Accepted Answer

Increased unconjugated bilirubin in plasma and increased urinary urobilinogen

Question 6

Which of the following best explains why lead poisoning can cause anemia and neurological symptoms?

Accepted Answer

Lead inhibits both ALA dehydratase and ferrochelatase, blocking heme synthesis and causing accumulation of toxic intermediates

Question 7

Which of the following is the correct sequence of events in the degradation of heme in macrophages?

Accepted Answer

Heme $\rightarrow$ biliverdin $\rightarrow$ bilirubin

Question 8

Which of the following enzymes requires pyridoxal phosphate (PLP) as a cofactor in the biosynthesis of specialized products from amino acids?

Accepted Answer

ALA synthase (in heme synthesis)

Question 9

A patient with acute intermittent porphyria (AIP) is likely to present with which of the following symptoms?

Accepted Answer

Abdominal pain and neuropsychiatric disturbances without photosensitivity

Question 10

Which of the following best describes the fate of bilirubin after it is conjugated in the liver?

Accepted Answer

It is secreted into bile, hydrolyzed and reduced by gut bacteria to urobilinogen, and then partly excreted in feces as stercobilin

Question 11

Which of the following is a distinguishing feature of obstructive (posthepatic) jaundice?

Accepted Answer

Pale, clay-colored stools and increased conjugated bilirubin in plasma

Question 12

Which of the following statements about creatinine is correct?

Accepted Answer

Creatinine excretion is proportional to total muscle mass and is a sensitive indicator of kidney malfunction

Question 13

Which of the following is the rate-limiting enzyme in catecholamine biosynthesis?

Accepted Answer

Tyrosine hydroxylase

Question 14

A patient with a deficiency in ferrochelatase would be expected to have an accumulation of which compound?

Accepted Answer

Protoporphyrin IX

Question 15

Which of the following is a function of histamine in the human body?

Accepted Answer

Acts as a chemical messenger mediating allergic and inflammatory responses

Question 16

Which of the following best explains why patients with porphyria cutanea tarda (PCT) develop photosensitivity?

Accepted Answer

Accumulation of porphyrins in the skin absorbs light energy, generating reactive oxygen species that damage tissues

Question 17

Which of the following is the main site of heme degradation in the body?

Accepted Answer

Mononuclear phagocyte system (especially liver and spleen)

Question 18

Which of the following amino acids is the precursor for serotonin synthesis?

Accepted Answer

Tryptophan

Question 19

A patient with chronic liver disease develops jaundice. Laboratory findings show increased conjugated and unconjugated bilirubin in plasma, dark urine, and pale stools. Which type of jaundice is most likely present?

Accepted Answer

Hepatocellular (hepatic) jaundice

Question 20

Which of the following best describes the role of cytochrome P450 monooxygenase in heme metabolism?

Accepted Answer

It is a heme-containing enzyme whose induction increases heme consumption and stimulates ALA synthase activity

Amino Acids: Conversion to Specialized Products and Heme Metabolism

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